Powdery mildew of Roses

Rose powdery mildew is a disease of roses caused by the fungus Podosphaera pannosa. The conspicuous white growth can affect all aerial parts of the plant, producing microscopic spores that spread the disease. High humidity is favourable for infection, and plants growing in areas where air movement is poor or the soil is dry can be severely affected.
Symptoms.
You may see the following symptoms:
A white, powdery fungal growth on the leaves and shoots. Upper, lower or both leaf surfaces can be affected
There may be discolouration (yellow, reddish or purple) of the affected parts of the leaf, and heavily infected young leaves can be curled and distorted
Mildew growth may also be found on the stems, flower stalks, calyces and petals
Heavily infected flower buds frequently fail to open properly
Mildew growth on stems (where it is often found surrounding thorns) and flower stalks is usually thicker and more mat-like than that on the leaves
The mildew growth on all parts may turn browner as it ages